The Mexican president, Enrique Peña Nieto, has declared three days of national mourning after the strongest earthquake to hit the country in a century left at least 65 people dead and destroyed thousands of buildings.

A major rescue operation is under way around the epicentre of the quake in the southern states of Chiapas, Tabasco and Oaxaca, where many are feared trapped under the rubble.

More than 600 aftershocks have been recorded since the earthquake struck off Mexico’s southern Pacific coast just before midnight on Thursday local time. Mexican officials have now upgraded the quake to 8.2 in magnitude.

The death toll was raised on Saturday, as more victims were registered in the poor southern states hardest hit by the disaster, authorities said.

Luis Felipe Puente, head of Mexico’s national emergency services, said three more bodies had been found in the state of Chiapas and another in Juchitán, the coastal town in neighbouring Oaxaca that suffered the greatest loss of life. A third of Juchitán’s homes collapsed or were badly damaged.

Mexico earthquake kills at least 61 and sparks mass evacuations

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Peña Nieto visited the town, which is located in one of the poorest parts of the country, on Friday. Many residents decided to spend the night outside for fear aftershocks would cause further damage.

Rescue teams were continuing to search for survivors with sniffer dogs and heavy machinery across the affected states.

In Juchitán, four people including two children were pulled alive from the rubble of a hotel in the centre of the city, but officials said they had also found many bodies.

“We are all collapsed, our homes and our people,” the Juchitán resident Rosa Elba Ortiz Santiago said. “We are used to earthquakes, but not of this magnitude.”

Peña Nieto said authorities were working to re-establish the water supply and provide food and medical attention to those in need, and that the government would help with reconstruction.

He asked people to avoid “slopes affected by the earthquake” in areas where Tropical Storm Katia has made landfall further north in Veracruz.

Power was cut to more than 1.8 million people after the quake, and authorities closed schools in at least 11 states to carry out safety checks.

The Mexican interior ministry said 428 homes were destroyed and 1,700 were damaged in Chiapas alone.

“Homes made of clay tiles and wood collapsed,” said Nataniel Hernández, a human rights worker living in Tonala, Chiapas, who worried that the bad weather could bring more structures down.

“Right now it is raining very hard in Tonala, and with the rains it gets much more complicated because the homes were left very weak, with cracks,” he said.

The quake triggered tsunami warnings and some high waves, but they did not cause any major damage. Authorities briefly evacuated a few residents of coastal Tonala and Puerto Madero because of the warning.

In Mexico City, windows were broken at the airport and power went out in several neighbourhoods, but the capital escaped major damage.

The tremor appears to have been stronger than the magnitude 8.0 earthquake in 1985 that levelled large parts of Mexico City, killing 5,000 people and destroying 10,000 houses.

Much of the capital is built on the soft soil of a former lake bed, making it vulnerable to earthquakes. Building codes have been tightened since 1985, and earthquake drills for apartment dwellers and office workers have become common.

Public officials were quick to provide updates on damage and give instructions, unlike in 1985 when the country’s politicians were nowhere to be seen and residents, many left homeless, fended for themselves and teamed up to pull people out of the rubble.

The Mexican seismological authority said the quake was 19km deep and triggered a series of magnitude 6 aftershocks.

“Chiapas is historically a very seismic state due to the interaction of five tectonic plates,” it said in a report on the earthquake. The state has suffered three tremors above magnitude 7 since 1970, including one on 7 November 2012 that measured 7.3.